The graphics are definitely better, but not a lot. There's more detail, more things happening and more characters on screen, and it all looks a bit crisper and more colourful, but it's not a major improvement, and certainly nowhere close to pushing Wii's graphic abilities.
...That concerns the new tracks, of course.
As for the re-used ones from the 'Cube, they're exactly the same.
Nintendo Network ID: Cheebee 3DS Code: 2320 - 6113 - 9046